Deep oil wells, which have not conventionally been regarded at all, and corrosive sour gas wells, the development of which was abandoned for a time, have recently been developed increasingly on a world scale to cope with increase of crude oil price and anticipated oil resource depletion in the near future. These oil wells and gas wells generally lie at great depths in a severe, corrosive environment of a high-temperature atmosphere containing corrosive substances, such as CO2 and Cl−. Accordingly, steel pipes for oil country tubular goods used for digging such an oil or gas well have to be highly strong and corrosion-resistant.
In general, highly CO2 corrosion-resistant 13%-Cr martensitic stainless steel pipes are used in oil wells and gas wells whose atmospheres contain CO2, Cl−, or the like. However, conventional martensitic stainless steels cannot wear in environments at high temperatures of more than 100° C. containing a large amount of Cl−. Accordingly, two-phase stainless steel pipes are used in oil wells requiring corrosion resistance. Unfortunately, the two-phase stainless steel pipes contain large amounts of alloying elements to reduce the hot workability. Consequently, they must be manufactured only by special heat treatment due to their reduced hot workability, and besides, they are disadvantageously expensive. Accordingly, an inexpensive 13%-Cr martensitic stainless steel-based pipe for oil country tubular goods having a superior hot workability and CO2 corrosion resistance has been strongly desired. On the other hand, oil well development in cold districts has recently become active, and, accordingly, superior toughness at low temperatures is often required in addition to high strength.
To these demands, improved martensitic stainless steels (or steel pipes) based on a 13%-Cr martensitic stainless steel (or steel pipe), having an enhanced corrosion resistance have been proposed in, for example, Japanese Unexamined Patent Application Publication Nos. 8-120345, 9-268349, and 10-1755 and Japanese Patent Nos. 2814528 and 3251648.
Japanese Unexamined Patent Application Publication No. 8-120345 has disclosed a method for manufacturing a seamless martensitic stainless steel pipe having a superior corrosion resistance. For a steel composition of a 13%-Cr martensitic stainless steel pipe, the C content is limited to the range of 0.005% to 0.05%, 2.4% to 6% of Ni and 0.2% to 4% of Cu are added in combination, and 0.5% to 3% of Mo is further added. Furthermore, Nieq is set at 10.5 or more. This steel material is subjected to hot working, subsequently cooled at air-cooling speed or more, and then tempered. Alternatively, after being cooled, the steel material is further heated to a temperature between AC3 transformation point+10° C. and AC3 transformation point+200° C., or a temperature between AC1 transformation point and AC3 transformation point, subsequently cooled to room temperature at air-cooling speed or more, and then tempered. According to this method, a seamless martensitic stainless steel pipe is achieved which has a high strength of the grade API-C95 or grater, corrosion resistance in environments at 180° C. or more containing CO2, and SCC resistance.
Japanese Unexamined Patent Application Publication No. 9-268349 has disclosed a method for manufacturing a martensitic stainless steel having a superior stress-corrosion cracking resistance to sulfides. In this method, a steel composition of a 13%-Cr martensitic stainless steel contains 0.005% to 0.05% of C, 0.005% to 0.1% of N, 3.0% to 6.0% of Ni, 0.5% to 3% of Cu, and 0.5% to 3% of Mo. After hot working and being left to cool down to room temperature, this steel material is heated to a temperature between (AC1 point+10° C.) and (AC1 point+40° C.) for 30 to 60 minutes, then cooled to a temperature of Ms point or less, and tempered at a temperature of AC1 point or less. Thus, the resulting steel has a structure in which tempered martensite and 20 percent by volume or more of γ phase are mixed. According to this method, the sulfide stress-corrosion cracking resistance is remarkably enhanced by forming a martensitic structure containing 20 percent by volume or more of γ phase.
Japanese Unexamined Patent Application Publication No. 10-1755 has disclosed a martensitic stainless steel containing 10% to 15% of Cr, having a superior corrosion resistance and sulfide stress-corrosion cracking resistance. This martensitic stainless steel has a composition in which the Cr content is set at 10% to 15%; the C content is limited to the range of 0.005% to 0.05%; 4.0% or more of Ni and 0.5% to 3% of Cu are added in combination; and 1.0% to 3.0% of Mo is further added. Furthermore, Nieq of the composition is set at −10 or more. The structure of the martensitic stainless steel contains a tempered martensitic phase, a martensitic phase, and a residual austenitic phase. The total percentage of the tempered martensitic phase and the martensitic phase is set in the range of 60% to 90%. According to this disclosure, corrosion resistance and sulfide stress-corrosion cracking resistance in environments where wet carbon dioxide gas or wet hydrogen sulfide is present are enhanced.
Japanese Patent No. 2814528 relates to an oil well martensitic stainless steel product having a superior sulfide stress-corrosion cracking resistance. This steel product has a steel composition containing more than 15% and 19% or less of Cr, 0.05% or less of C, 0.1% or less of N, 3.5% to 8.0% of Ni, and 0.1% to 4.0% of Mo, and simultaneously satisfying the relationships: 30Cr+36Mo+14Si−28Ni≦455(%); and 21Cr+25Mo+17Si+35Ni≦731(%). According to this disclosure, the resulting steel product exhibits a superior corrosion resistance in severe environments in oil wells where chloride ions, carbon dioxide gas, and a small amount of hydrogen sulfide gas are present.
Japanese Patent No. 3251648 relates to a precipitation hardening martensitic stainless steel having superior strength and toughness. This martensitic stainless steel has a steel composition containing 10.0% to 17% of Cr, 0.08% or less of C, 0.015% or less of N, 6.0% to 10.0% of Ni, 0.5% to 2.0% of Cu, and 0.5% to 3.0% of Mo. The structure of the steel is formed by 35% or more cold working and annealing and it has a mean crystal grain size of 25 μm or less and precipitates with a particle size of 5×10−2 μm or more in the matrix. The number of the precipitates is limited to 6×106 per square millimeter or less. According to this disclosure, a high-strength precipitation hardening martensitic stainless steel in which toughness degradation does not occur can be achieved by forming a structure containing fine crystal grains and less precipitation.
However, improved 13%-Cr martensitic stainless steel pipes manufactured by the techniques of Japanese Unexamined Patent Application Publication Nos. 8-120345, 9-268349, and 10-1755 and Japanese Patent Nos. 2814528 and 3251648 do not stably exhibit desired corrosion resistance in severe, corrosive environments at temperatures of more than 180° C. containing CO2, Cl−, or the like.
In view of the circumstances of the known arts stated above, this disclosure has been achieved. The object of this disclosure is to provide an inexpensive, corrosion-resistant stainless steel pipe for oil country tubular goods, preferably a high-strength stainless steel pipe for oil country tubular goods, having a superior hot workability and exhibiting a superior CO2 corrosion resistance even in severe, corrosive environments at temperatures of more than 180° C. containing CO2, Cl−, or the like.